Laboratory work for the coming year will be devoted to studies of improved techniques of immunosuppression for cardiac allografting. This work will represent a continuation of promising areas of previous work in this laboratory. The use of high-dose pulse cyclophosphamide therapy for tolerizing the recipient of a cardiac allograft will be further investigated. When our toxicity studies on cyclophosphamide are complete, a three-day high- dose regimen will be evaluated in the immediate post-graft period in canine recipients of orthotopic heart grafts. Study will continue on the mechanism of action of chicken antirat lymphocyte globulin (CARLG). Our current hypothesis is that this agent acts by binding to donor histocompatibility antigen with the antigen-antibody complex functioning as an immunoregulatory signal. Studies involving perfusion of the organ graft prior to implantation and studies involving injections of antigen-antibody complexes will be evaluated in the rat heterotopic cardiac allograft system. Our previous pilot study with chicken antidog lymphocyte globulin (CADLG) in canine cardiac allograft recipients yielded negative results. In retrospect this was at least in part due to inadequate dosage of the agent. With our present understanding of the dose requisite for immunosuppression in the rat, we will again attempt to adapt this new immunosuppressive agent to orthotopic canine cardiac allografts. Finally, a study involving antigen pretreatment of cardiac canine allograft recipients with donor antigen is in progress. Previous studies in this laboratory have shown that presensitization can occur with soluble antigen pretreatment of graft recipients. We propose to obviate this eventuality by complexing the injected antigenic material with Fab pieces of alloantibody. It is our postulate that this complex will function as an immunoregulatory signal and will permit the induction of tolerance without risk of presensitization.